The present invention generally relates to high intensity discharge (“HID”) lamps, arc tubes, and methods of manufacture. More specifically, the present invention relates to HID lamps, arc tubes, and methods of manufacture wherein the pressure of the fill gas in the arc tube is greater than about one-half atmosphere at substantially room temperature.
Short arc gap metal halide lamps are particularly suited for fiber optic lighting systems, projection display, and automotive headlamps. Metal halide lamps with high pressure fill gas have been favored in many applications because of the fast warm-up, relatively long life, and relatively high efficiency in producing white light with good color rendition.
In the manufacture of such lamps, it is desirable to obtain a final fill gas pressure which is greater than one atmosphere at substantially room temperature. Final fill gas pressures greater than about five atmospheres are common and fill gas pressures may be as high as about two hundred atmospheres.
In the manufacture of metal halide lamps, it is known to obtain a superatmospheric fill gas pressure by freezing an amount of the fill gas (heretofore xenon) into the light emitting chamber of the lamp prior to sealing the chamber. The volume of gas frozen into the chamber (when at substantially one atmosphere and room temperature) is larger than the volume of the chamber so that the pressure of the gas sealed within the chamber is greater than one atmosphere when the temperature of the gas returns to substantially room temperature. The pressure of the fill gas sealed within the chamber at substantially room temperature equals the ratio of the volume of gas frozen into the chamber (at substantially one atmosphere and room temperature) relative to the volume of the chamber.
In the manufacture of superatmospheric arc tubes, it is difficult to control the amount of fill gas contained in the sealed arc tube due to the difficulty in preventing the escape of fill gas from the arc tube during the sealing process when the open tubular end portion of the arc tube is heated to about 2000° C. prior to pinch or shrink sealing the end portion.
Applicant has discovered a novel method for making superatmospheric arc tubes containing a fill gas such as xenon or krypton wherein the amount of the fill gas contained in the arc tube may be precisely controlled.